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2 Imagine you are riding in a car…How do the objects that are close to you (trees, telephone poles, etc) seem to move relative to things far in the distance?Copernicus (1500s) noticed that some objects in the night sky seemed to move much more quickly than others.Q: What do you think the quicker moving objects were?A: Planets

3 Copernicus was right!The stars are much farther away from Earth than the planets!They are so far away that a new unit of length – the light year – was created to measure such huge interstellar distances.

4 Just how big is the universe?Light year = the distance light travels in a yearSpeed of light = 186,000 miles/secThat’s like traveling fromNew York to San Francisco 62 times in one second!Light year = 6,000,000,000,000 (6 trillion) miles per year

5 The closest star to Earth (other than our sun) is Proxima Centauri, it is 4.3 light years (26 trillion miles) away.This is the same distance as walking around the Earth more than 944 million times!Space probes travel into space at an astounding speed of over 80,000 miles/hourAt this speed, it would take a space probe 150,000 years to reach Proxima Centauri.

6 From Ohio, we cannot see Proxima CentauriFrom Ohio, we cannot see Proxima Centauri. It can only be seen from the southern hemisphere.The brightest star in the northern hemisphere (other than our sun) is Sirius.Sirius is referred to as the “dog star” because it is in the constellation Canis Major (big dog).It is 8.6 light years away from Earth.

7 From Home Plate to 10 Million Light-Years AwayLook at the illustration on page 569 in your text book.

8 Interesting Fact!The Hubble Space Telescope has taken detailed images of distant galaxies.These images have led astronomers to think that there may be 10 times the number of galaxies in the universe than previously thought!Are we alone?

9 Why do some stars seem brighter than others?“Star A” and “Star B” are the same size, brightness, and distance from Earth.Q: When looking at these stars from Earth, which star will appear to be brighter?A: They will appear to be the same brightness.

10 Why do some stars seem brighter than others?“Star A” and “Star B” are the same size and brightness.“Star A”, however, is much farther away from Earth than “Star B”.Q: When looking at these stars from Earth, which star will appear to be brighter?A: “Star B” will appear brighter just because it is closer to Earth.

11 Why do some stars seem brighter than others?“Star C” is very big and bright.“Star D” is not so big nor bright.Q: How could these two very different stars seem to be the same brightness from Earth?A: “Star C” is much farther away from Earth, thus, it seems as though it is the same brightness, or magnitude, as “Star D”.

12 The Birth of a Star Stars undergo a type of “life cycle”The stages of the life of a star generally follow a regular pattern and are predictableThe stages are similar to us in that the stars life cycle has a birth, infancy, maturity, old age, and death

13 BIRTH Stars begin their lives as clouds of dust and gas called nebulaeClouds of gas and dust are drawn together by gravity creating a protostar

14 INFANCY as a Protostar Gives off no visible lightMust undergo many changes before it is recognized as a star.It continues to contract over a period of millions of yearsShrinkage causes an increase in pressure and temperature

16 MATURITY as a Main Sequence StarThe star will spend most of its life in this stable phase.The life span of a star depends on its size.Very large, massive stars burn their fuel much faster than smaller stars. Their main sequence may last only a few hundred thousand years.Smaller stars will live on for billions of years because they burn their fuel much more slowly.Our sun is a “medium” size star.

17 OLD AGE Eventually, the star's fuel will begin to run out.It will expand into what is known as a red giant.Massive stars will become red supergiants.This phase will last until the star exhausts its remaining fuel.The star will collapse.

18 DEATH of an Average Size StarMost average stars will blow away their outer atmospheres to form a planetary nebula.Their cores remain behind and burn as a white dwarf until they cool down.What is left is a dark ball of matter known as a black dwarf.

19 DEATH Of an Very Large StarIf the star is massive enough, the collapse of the star will trigger a violent explosion known as a supernova.Supernovas are very bright and can cause a brief (few months) burst of radiation that can outshine an entire galaxy.During this explosion, a supernova can give off as much energy as our Sun could emit over its life span!

20 DEATH Of an Very Large StarDepending upon how large the star was to begin with, the star with collapse further to become either a neutron starORit will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole.